In both Amiga and DOS Lemmings, there is a "floor check" that works exactly like this: before builder steps onto the 6-pixel build brick it laid down, it first checks the two pixels marked "x" below immediately above the row of 6 pixels that is the build brick. If either one has ground (ie. not air) then it stops without stepping up and turns around instead. It's actually an additional check from the "ceiling check" which involves checking a single pixel more or less right above lemming's head. The obvious intent of the floor check is so that when lemming is on a steep enough upward slope (ie. steeper than the slope of the build bridge), it'd stop properly before the bridge would wind up going into the slope.
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BBBBBB(B = pixels of brick laid by builder, x = the 2 pixels checked by floor check, . other pixels ignored by floor check).
NeoLemmix has an even stronger floor check that check for more pixels, so that's why in your NeoLemmix video, the bridges don't even have to really touch each other (the corners of pixels don't count as touching) to turn the builder around.
On DOS and Amiga Lemmings, to turn the second lemming around like in NeoLemmix without using an extra builder like you do, the two bridges must be positioned closer so that they are fully touching one another, so that the right tip of second brick of the left bridge would reach into the left "x" relative to the first brick of the right bridge. This requires increasing the release rate from the initial 50 so that the second lemming can step off the first lemming's bridge before the second brick of that bridge has been laid down.
Attached is an example of how to set up the crowd-blocking gadget in DOS or Amiga Lemmings using only 5 builders, although 2 lemmings will overtake the workers before the gadget is ready so you'll need to floater those. You'll need to view the LRB replay file using the old LemmixPlayer (which faithfully replicates nearly all the DOS game's mechanics) as detailed
here. The methods you found for Amiga/SuperLemmini are probably a little easier to execute than this 5-builder method, even if consuming more builders.
When I originally reported a 100% solution for the level on DOS Lemmings, I actually did not rely on this trick at all. Instead I relied on the thing you observed on the other thread, that involves stacking 2 builders to create steps of height 2 pixels, and then have other lemmings build into such steps to turn around. I've attached this method for reference in the second LRB replay file. I suspect this method can even work as-is on NeoLemmix and WinLemm (but have not tested either).
I haven't played WinLemm much so I'm not versed on how its checks differ. But there has to be some form of floor check in play even if the pixel-precise details may differ; without some kind of a floor check, on a steep enough upward slope the builder would go a bit further into the slope before there are pixels of ground high enough to trigger the ceiling check and stop the builder.
One side effect of the rather lazy checks in the original games, is that there are actually no checks for any of the pixels between the feet (floor check) and head (ceiling check) of the lemming! That's why on levels like "King of the Castle", if you can get the lemming high enough for its head to protrude above the top of one of those thin platforms, then when you assign it builder it will be able to keep building up uninterrupted until the bricks start touching the bottom of the platform.
